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Fixes pertaining background color and grayscale conversions #270

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Fixes pertaining background color and grayscale conversions #270

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Commits on Jan 10, 2019

  1. Fix processing of background color when converting to grayscale 

    When processing transparent pixels (with png_set_strip_alpha) while
converting to grayscale (with png_set_rgb_to_gray), libpng failed to
convert the background color specified to the user to grayscale as
well. This caused the documented pattern of handling bKGD chunks to
fail for this corner case.

Note that in case of conversions in the opposite direction (grayscale
to RGB), the background color was already properly converted to RGB.

This patch fixes this inconsistency, and adds a test program to the
test suite to verify the fix.
    @CyberShadow
    CyberShadow committed Jan 10, 2019
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  2. Fix expanding low-bit grayscale background color 

    When expanding low-bit (4-bit and lower) images to 8 or 16 bits, the
background color (as set by png_set_background) only had its
red/green/blue fields populated to the correctly expanded values, but
the gray values were erroneously left at their unexpanded values. This
caused transparency processing (as enabled by png_set_strip_alpha) to
use the unexpanded background color when substituting transparent
pixels.

Rectify this by updating the .gray fields of the background color, and
add a test.
    @CyberShadow
    CyberShadow committed Jan 10, 2019
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